There are various applications in which electrical currents must be passed between two opposed faces which are clamped together or biassed towards each other by spring force or gravity or the like, a particular and important example being the feeding of electrical current to an electrode.
Clamps for cylindrical graphite electrodes, such as are used on an electric arc furnace, are normally made from cast or wrought copper and often include passages for water cooling.
In recent years, and in order to reduce the consumption of the graphite electrode, a technique has been developed of coating the graphite with aluminium and a large number of electric arc furnaces have been converted to use the aluminium coated type of electrodes. This has produced the result that, whereas previously a direct copper contact was satisfactory between the clamp and the electrode, a different system had to be devised for the aluminium coated electrode, because it is impracticable to make contact between copper and aluminium, particularly in hot conditions.
For this reason the present practice is to provide the electrode clamp with two graphite pads to give the required electrical contact between the clamp and the coating of the electrode.
This has given rise to disadvantages in that the large surface areas of both the curved and the flat faces demand a high degree of dimensional and angular accuracy which is vital in order to achieve good electrical and thermal transfers. In practice it is found that the tolerances of the various parts cannot be kept within such strict limits as to ensure good contact surfaces. Furthermore, it is difficult to ensure dirt is not trapped between the copper and the graphite insert. This in turn results in a poor transfer of heat and in arcing, and this in turn damages the copper electrode clamp.
Another type of connection is to be found in the electro slag refining process, where the electrode is constituted by a steel billet. Electric current is fed to this billet through a stainless steel pad which is welded onto the top of a stainless steel bar which in turn is welded to the top of the billet. The stainless steel pad is supported on a fork which carries the weight of the assembly and through which the vertical height can be controlled so that an arc is maintained on the bottom of the electrode.
At the present time, in order to connect one phase of the electrical supply to each of the three billets representing the three electrodes, a copper stub clamp is used which is shaped to form two-line contacts with the top face of the stainless steel pad and which is mechanically loaded with approximately nine tons to maintain the necessary electrical contact between the copper stub clamp and the steel pad and also to retain the pad securely on the fork. Owing to the relatively low conductivity of stainless steel, it is often found that when a large current is applied between the line-contacts of the stub clamp and the surface of the stainless steel pad, there is local over-heating and subsequent damage to the contact faces.